Collection of Collections Is a Code Smell

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Have you run into, or coded yourself into a place where the obvious solution was to create a HashMap of HashMaps? How about a HashMap of ArrayList or come other combination of collection of collections. While some things, such a matrices, are naturally represented this way, more often than not, creating a collection of collections is an indication of a missing design element. It is because of tendency that the coding pattern would be considered a "code smell".

The term "code smell" was first coined by Kent Beck. It is used to describe code that is awkward looking or doesn't look right. It is the awkwardness that tends to point to some deeper underlying problem. In the example above, the code smell is often an indication that one needs an object to more naturally represent a relationship between the keys of the first outer collection and the values of the inner collections. This is perhaps best seen in a small example such as that provided in listing 1.

public class AllPersons {

   private HashMap<String,HashMap> allPersons = new HashMap<String,HashMap>;
   public void addPerson( Person person) {
       HashMap<String,HashMap> persons = this.allPersons.get( person.getLastName());
       if ( persons == null) {
           persons = new HashMap<String,Person>();
           this.allPersons.add( person.getLastName(), persons);
       persons.add( person.getFirstName(), person);


Listing 1. AllPersons, an implied collection.

This class AllPersons is an implied collection that is keyed on a persons last name. The inner collection is then keyed on the persons first name. The code smell in the example is that we are keying on the last name and then the first. The question is, what is the missing design element if there is one.

When we use a HashMap we are implicitly creating an index much in the same way we'd create an index in a database table. If we were to create an index on a single column, that would be a simple key. If we combine two or more simple keys to create another index, we have created a compound key. And this is exactly what we are doing in this example, creating an index based on two fields. From this we can conclude that the missing design element is a compound key. Listing 2. demonstrates the code with our newly discovered class.

public class AllPersons {

   private HashMap<CompoundKey,Person> allPersons = new HashMap<CompoundKey,Person>;
   public void addPerson( Person person) {
       this.addPerson( new CompoundKey( person.getFirstName(), person.getLastName(), person);


Listing 2. AllPersons, an implied collection using a CompoundKey.

Though it is perhaps a bit difficult to see in this short example, the new version will contain far less code than the original. This implosion in the code base is typical when one improves the design. Not only do we have less code to read, the code is more readable in that CompoundKey has a specific meaning in our domain that clearly communicates purpose.

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